I have long had trouble remembering the tone of 摸 (mō; to touch). One of the reasons may be that the following two other characters, which have formal or implied meanings of "to rub" (sort of close to "to touch" in meaning), both have mó as their Pinyin:
磨
摩
I normally remember the tones for those two characters. They are parts of multi-character words which are firmly in my vocabulary, and of which the pronunciations of all syllables come easily to my mind:
摩/磨擦 (mócā; friction)
按摩 (ànmó; to massage)
However, the most common usage for me of 摸 is the much less distinctive:
摸不到 (mō bù dào; can't reach [some physical object])
The last two characters 不到 commonly follow a wide variety of verbs, in all cases (that I can imagine) with a similar meaning of not being successful at something. Being so generically and commonly used, those two characters don't facilitate (for me, anyway) remembering the tone of 摸 or of any other verb placed before them.
In any case, my new way to remember that 摸 is first tone:
Extend the idea of 1 being less than 2 to the characters' tones -- a touch (摸) is lighter ("less") than a rub (磨/摩).
